Hi Heikki,
Heikki Linnakangas wrote:
That's really exciting news!
I'm sure you spent a lot of time tweaking the settings, so let me ask
you something topical:
How did you end up with the bgwriter settings you used? Did you
experiment with different values? How much difference did it make?
Background writer is still a pain to get it right.. I say it is a
necessary evil since you are trying to balance it with trying to level
writes to the disks and lock contentions caused by the writer itself to
the postgresql connections. Our typical problem will arise at the high
number of users where all users are suddenly locked due to the bgwriter
holding the locks.. Using the hotuser script (which uses pearl/Dtrace
combination) we ran quite a bit of numbers trying to see which ones
results in less overall time spent in PGLock* calls and yet gave good
uniform writes to the disks. After reaching the published settings,
everynow and then we would try playing with different values to see if
it improves but generally seemed to degrade if changed.. (Of course your
mileage will vary depending on config, workload, etc).
Still I believe the locking mechanism needs to be revisited at some
point since that seems to be the one which will eventually limit the
number of users in such a workload. (Specially if you dont hit the right
settings for your workload)
Hopefully soon we will get access to bigger capacity servers and redo
SMP tests on it with the background writer.
Regards,
Jignesh
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